Electric cartridge fuses



Oct. 25, 1966 E. sALzER ETAL 3,281,556

ELECTRIC CARTRIDGE FUSES Filed March 30, l964 2 Sheets-Sheet l IO l5 20 25 30 MINIMUM FUSING CURRENT, AMP;

INVENTORS www ALzER/ FREDHMCK J. KOZACKA @Y ATTY.

Y" A 5 K L R C A 6 n O A 5 t i. W 5, Nw W2 E R Mn. 1A .n V E 2, N M K 3 .m S Ww S .N 2 .dm D M n WM WDW L mw v\ T E E .Du Y R M E m u m AM m .v s m M/L E E .H, L E f nv A m YW.. m m

a d 'l' United States Patent O This invention relates to electric cartridge fuses, and more particularly to high-interrupting-capacity currentlimiting cartridge fuses.

It is a general object of this invention to provide a new and improved kind of electric cartridge fuses.

Another object of this invention is to provide cartridge fuses including casings of an inorganic material which cartridge fuses are free from certain limitations and drawbacks inherent in prior art cartridge fuses having casings of an inorganic material.

Another object of this invention is to provide electric cartridge fuses having casings of an inorganic material which is relatively inexpensive, resistant against explosion-like pressure surges, temperature and heat-'shock resistant, dimensionally stable and machineable to close tolerances.

Fiber as well as various other kinds of cellulosic materials are widely used for casings of electric fuses. Such materials are, however, unsatisfactory on account of their lack of dimensional stability. Casings made of such materials also have but a very limited ability to withstand elevated temperatures resulting from protracted currents in the range of the minimum fusing current, or resulting from currents slightly above that range. Nor are casings made of such materials capable of withstanding immediate physical engagement with an incandescent fulgurite resulting from fusion of an arc-quenching quartz sand filler.

It is, therefore, another object of this invention to provide cartridge fuses having casings not subject to the aforementioned limitations and/or drawbacks.

The advent of modern high-interrupting-capacity fuses including fuse links of high current-limiting action metals and including quartz sand as arc-quenching medium called for and resulted in the development of fuses having casings of materials other than fiber, or any cellulosic materials of another kind. Modern high-interrupting-capacity fuses including fuse links of a high current-limiting action metal and including quartz sand as arc-quenching medium are generally provided either with casings of a syntheticresin-glass-cloth laminate, or with casings of a ceramic material. The former type of casing material is widely applied in this country, while the latter type is preponderant in Europe. Both types of materials are subject to severe limitations and/ or drawbacks.

It is, therefore, another object of this invention to provide cartridge fuses including fuse links of a high currentlimiting action metal, and including quartz-sand as arcquenching medium, having casings not subject to the limitations and/ or drawbacks of synthetic-resin-glass-cloth laminates and not subject to the limitations and/ or draw backs of ceramic materials.

The main objection against the use of synthetic-resinglass-cloth laminates is their relative high cost. From a mechanical point of view melamine-glass-cloth laminates appear to be among the most desirable synthetic-resinglass-cloth laminates for casings of high-interrupting-capacity fuses. These particular synthetic-resin-glass-cloth laminates have, however, a relatively low continuous temperature withstand ceiling. Fuses having casings of such laminates often call for an extra large volume of quartz sand for thermal protection of their casings, or for other equivalent means preventing thermal damage to the casings. Casings made of silicone-glass-cloth laminates have a far higher temperature ceiling than casings of melamine- ICC glass-cloth laminates, but the mechanical properties of the former are not quite as satisfactory as those of the latter.

It is, therefore, another object of this invention to provide cartridge fuses including fuse links of a high currentlimiting action metal, and including quartz-sand as arcquenching medium, having casings which involve smaller production cost than casings of synthetic-resin-glass-cloth laminates, and are capable of withstanding higher temperatures for protracted periods of time than most synthetic-resin-glass-cloth laminates.

Casings of fuses of ceramic materials tend to be mechanically relatively fragile and are liable to crack under severe heat shock incident to severe interruptions. Another serious limitation of casings of ceramic materials, in particular porcelain and steatite, is that the tolerances thereof are undesirably large, and that they cannot be machined. Virtually all U.S. designs of low voltage fuses are predicated on the use of fabricated machineable casings. The aforementioned numerous disadvantages of ceramics resulted in the development, in this country, of fabricated casings of synthetic-resin-glass-cloth laminates.

It is therefore, another object of this invention to provide electric cartridge fuses including fuse links of a high current-limiting action metal, and including quartz-sand as arc-quenching medium, having casings which are not subject to the aforementioned limitations and drawbacks of casings of ceramic materials.

Another object of the invention is to provide high-interrupting-capacity fuses whose links operate at relatively high temperatures when carrying the rated current, or the minimum fusing current, thereof.

Still another object of this invention is to provide highinterrupting-capacity fuses which are considerably more compact than required by the Underwriters Fuse Standard.

Other objects of the invention and advantages thereof will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the invention reference may be had to the following description thereof taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the concept of high current-limiting action metals;

FIG. 2 is a longitudinal section of a fuse embodying this invention taken along 2-2 of FIG. 2A;

FIG. 2A is a longitudinal section of the fuse of FIG. 2 4taken along 2A-2A of FIG. 2;

FIG. 2B is a transverse section of the fuse of FIG. 2 taken along 1A1A of FIG. 2;

FIG. 3 is a longitudinal section lof another fuse embodying this invention taken along 33 of FIG. 3A;

FIG. 3A i-s a longitudinal section of the fuse of FIG. 3 taken along 3A-3A of FIG. 3;

FIG. 4 is a longitudinal section of still another fuse embodying this invention taken along 4-4 of FIG. 4A;

FIG. 4A is a longitudinal section of the fuse of FIG. 4 taken along 4A-4A of FIG. 4; and

FIG. 4B is a transverse section of the structure of FIG. 4 taken along 4B-4B of FIG. 4A.

The degree of suitability of a particular metal to be used for form-ing fuse links in high-interrupting-capacity current-limiting fuses depends upon a number of parameters including the conductivity of the particular metal, its latent heat of fusion, etc. A plot such as that of FIG. 1 is indicative of the degree of suitability of metals to be used for forming fuse links in fuses of the above description. FIG. l refers to round wires of various metals and shows peak let-through currents plotted against minimum fusing currents. The members of Ithe family of curves shown in FIG. l have been designated by the symbols Sn, Zn, Al, Cu, A-g, thus indicating the metal to which each pa-rticular curve refers. It is :apparent that the peak letthrough currents of fusible elements having the same minimum fusing current vary widely depending upon the metal of which the fusible element is made. The smaller the peak let-through current for a given minimum fusing current, the higher the degree of suitability of .the particular metal to be used for forming fuse links in highinterrupting-capacity current-limiting fuses. It is evident from lFIG. 1 that the metals Ag and Cu are a class of metals having a far higher degree of suitability for the purpose in hand than Al, Zn or Sn. Silver and copper form a separate class of metals as far as their suitability is concerned of being used for forming fuse links in highinterrupting-capacity current-limiting fuses. Hence silver and copper may rightfully be referred to by the generic tenm high-current-limiting .action metals.

v The structure of FIGS. 2, 2A and 2B comprises ribbon link 1 of a high-curren-t-limiting action metal provided with six perforations forming six serially related points 2 of reduced cross-section. Ribbon link 1 extends along the axis of a tubular casing 3 of a dense machineable precompressed or densied mixture of a hydraulic cement and inorganic fibers. Slotted washers 4 are arra-nged on both ends of casing 3 and fuse link 1 is threaded through the slots in washers 4 and its ends are bent 90 degrees as clearly shown in FIG. 2A. Terminal elements or terminal caps 6 are mounted on the ends of casing 3. A thin layer 7 of solidified soft solder occupies the space between washers 4 and Iterminal caps 6. Layer 7 minimizes the resistance between fuse link 1 `and caps 6 which -a-re conductively interconnected by fuse link 1. Fuse link 1 is surrounded by a body of quartz sand to which reference character l8 has been applied. The terminal caps 6 are secured to casing 1 by a plurality of transverse pins 9 projecting into radial bores provided in casing 3. These bores are machined, i.e. drilled, into casing 3 and pins 9 are firmly held therein by frictional engagement without resorting to `a bonding agent, or a bonding paste. When the fuse of FIGS. 2, 2A and 2B carries current the casing 3 thereof may reach relatively high temperatures, i.e. much higher temperatures than admissible in case of casings made of a synthetic-resin-glass-cloth laminate. This may require the Iuse of a solder for layer-s 7 which has a relatively high fusing point, e.g. of a silver cadmium solder, or of a hard solder instead of a soft solder. Since the material of which casing 3 is made is capable of withstanding for protracted periods of time much higher temperatures than organic materials, a higher current rating may be assigned to fuses of the type shown in 'FIGS 2, 2A and 2B than to comparable fuses having about lthe same size b-ut including a casing of an organic material, e.g. a synthetic resin.

Fuse link 1 is preferably of silver and adapted to interrupt the current path between terminal caps 6 only when the fusing point of silver is reached at any of its points 2 of reduced cross-section. This adaptation is simply achieved by omitt-ing any means which cause the temperature at which the current path between caps 6 is severed, to be reduced below the fusing point of silver. Thus fuse link 1 is not associated with any link-severing overlay, or similar link-severing means, of a metal which has a llower fusing point than silver.

As mentioned above, layers 7 must be made of metal which has a relatively high fusing point. Since the current density in layers 7 is much less than the current density at the points 2 of reduced cross-section of fuse link 1, and since the cooling of layers 7 is far more effective than that of the points 2 of reduced cross-section of fuse link 1, the tempera-ture of the points 2 of reduced crosssection will always be considerably higher than that of layers 7.

The hottest point of the fuse link is situated approximately at the center thereof. At protracted overloads this point may reach temperatures-and be maintained for long periods of time at temperatures-which are just below the fusing point of silver. Since the arc-quenching `filler 8 consists of quartz sand to achieve high arc voltages, and since quartz sand is la relatively good conductor of heat, the radial temperature gradient across filler 8 is small. Under such conditions fuses having casings of a synthetic resin must be extremely bulky, since the spacing between the fuse link and the casing must be relatively large to protect the casing from thermal deterioration.

In the structure of FIGS. 2, 2A and 2B the lateral walls of the casing are arranged so close to fuse link 1 that the center of said lateral wall exceeds deg. C. when fuse link 1 is carrying the minimum fusing current for the period of time required to cause fusion of fuse link 1. This is the proposed degree of minimum compactness. The compactness of the structure may be increased inasmuch as, and to the extent that, higher temperatures at the center of the lateral wall of casing 1 are tolerable in each particular application. There is practically no temperature ceiling as far as the withstand temperature of the material is concerned of which casing 3 is made. The preferred casing material is a dense mixture of Portland cement and asbestos fibers formed under high pressures, i.e. pre-compressed or densified asbestos cement. This material may safely be subjected continuously to temperatures up to, and even in excess of, 360 deg. C. That is ample for any forseeable high temperature application.

While the fuse structure of FIGS. 2, 2A and 2B is a rapidly acting fuse, the structure shown in FIGS. 3 and 3A is a time-lag fuse. `The structure of FIGS. 3 and 3A comprises a pair of blade contacts 10 having axially inner ends conductively interconnected by a pair of fuse links 11 of a high current-limiting action metal (silver, copper). Time-lag is being achieved by providing fuse links 11 with link-severing overlays 12 of tin, or another metal having a relatively low fusing point. Casing 13 housing fuse links 11 is closed by a pair of asbestos washers 14 and a pair of terminal caps or ferrules 15. Blade contacts 10 project from the outside of casing 1 through rectangular cut-outs or apertures in caps 15 and lwashers 14 into casing 1. The latter is filled with a body of quartz sand 16. Resilient hollow pins 17 project transversely through housing 13 and blade contacts 10 and are enlarged at their axially outer ends by drive screws 18 which are pressed into them. Housing 13 is made of a precompressed or densilied mixture of hydraulic cement and inorganic fibers. The holes for receiving hollow pins 17 are drilled into casing 13.

The temperature rise of fuse link 11 is limited by the presence of tin overlay 12. The fact that casing 13 is made of precompressed or densified asbestos cement tubing makes it possible to impart much smaller dimensions to the fuse structure of FIGS. 3 and 3A than those specified in the Underwriters Standard for Fuses, and to use quartz sand as an arc-quenching filler without irnpairing the casing 13 by the hot fulgurites resulting from fusion of the quartz sand at the occurrence of severe short-circuit current-s. The structure of FIGS. 4, 4A and 4B is intended for higher voltage ratings, i.e. voltage ratings in the order of several kv. Casing 19 is formed by a length of precompressed dense asbestos cement closed at both ends thereof by terminal caps 20. The latter are conductively interconnected by the multiperforated fuse link 21 of silver. Both ca-ps 20 are perforated and fuse link 21 projects through the perforations in caps 20 into recesses on the outside of caps 20. These recesses are filled with pools 22 of solidified solder conductively connecting fuse link 21 to caps 20. Casing 19 is filled with a body 23 of quartz sand. Caps 20 are pinned to housing 19 by means of pins 24 frictionally secured in radial bores formed in housing 19.

Mixtures of inorganic fibers and of hydraulic -cements are widely used in the building trade, and for manufacturing ovens, or for providing thermal insulation for liuid carrying pipes. Such mixtures generally have not the combination of all the properties required for :application as fuse casings which include mechanical strength in the presence of explosion-like pressure surges, temperature shock resistance, surface hardness, etc.

Precompressed mixtures of inorganic fibers and of hydraulic cements, in particular of Portland cement, have much better mechanical properties than other kinds of hydraulic cementitious material-s having fillers of inorganic reinforcing fibers. Even so the tensile strength of precompressed or densified fiber-reinforced cement is less than that of any other inorganic material heretofore successfully used for casings of electric cartridge fuses. Thus the tensile strength of precompressed Portland cement reinforced by moderately long asbestos fibers is in the order of 1400 p.s.i., whereas the tensile strength of the grade of porcelain used as a material for casings of electric fuses as well as the tensile strength of steatite as used as a material for casings of electric fuses is far higher.

We have found, however, by appropriate tests both overload tests and short-circuit tests-that precompressed cementitious fiber-reinforced materials lends themselves well to be used for casings of electric cartridge fuses provided that certain precautions are taken, and in view of the facts which are set forth below more in detail.

If an arc-quenching filler of pure quartz sand is used, the generation of internal pressure is minimized, and this is a prime requirement for the use of casings of precompressed cementitious fiber-reinforced materials.

The generation of pressure depends upon the degree of current-limiting action. The larger the degree of current-limiting action of a given fuse link structure, the better the particular fuse link structure is adapted to be housed in a casing of a precompressed cementitious material which is reinforced by mineral fibers. Precompressed cementitious casings reinforced by mineral bers lend themselves particularly well to fuse structures having a current-limiting ratio of less than 15. Currentlimiting ratio is the ratio of the value of the current allowed to iiow through the circuit at blowing times of 0.01 sec. to the rated current of the fuse. The rated current is deemed equal to 135% of the minimum fusing current.

Precompressed cementitious materials are pressureshock and temperature and heat-shock resistant and do not crack if physically engaged by an incandescent fulgurite. This makes it possible to dispense with'layers of quartz sand interposed between the fulgurite and the casing for thermal protection of the latter. The space so gained may be used to increase the wall thickness of the casing and hence the mechanical strength thereof to better withstand the a-ction of internal pressure.

Every current-limiting fuse designed for a predetermined circuit voltage dissipates a maximum of arc energy and generates the highest internal pressure at a predetermined available short-circuit current. Hence the highest pressures are impact pressures of an explosion type rather than relatively steady pressures such as applied to determine the tensile strength of materials. Precompressed cementitious casings reinforced by inorganic or mineral bers have a relatively high degree of mechanical shock resistance.

A further point of interest resides in the fact that the specific heat of precompressed cementitious fiber-reinforced materials is relatively high, causing effective cooling of the fulgurites resulting from fusion of the arcquenching quartz sand liller.

The sizes of fuses specied in the Underwriters Standard for Fuses have been determined with gas evolving arcquenching iillers in mind rather than a non-gas-evolving ller such as quartz sand. Substitution of quartz sand for a gas evolving arc-quenching filler makes it possible .the conventional casing materials to reduce the sizes of fuse structures and to reduce the mechanical strength requirements for the casings, particularly if the current-limiting action of the fuse link structure is considerable.

Precompressed fiber-reinforced Portland cement may be made in sheet form by conventional processes. Glass fiber-s as well as asbestos fibers may be used as filler material, and the liber content may be varied within limits. Good results may be obtained by mixing asbestos fibers and Portland cement in approximately equal portions by weight. A sufiicient quantity of water is then added to form a thick or creamy slurry. This slurry is then formed into sheets which are stacked in a hydraulic press. A flat solid metal plate is arranged above each sheet of asbestos cement and a water permeable sheet, eg. a metal screen, is arranged below each sheet of asbestos cement. Thereupon this composite stack including asbestos cement sheets separated by solid metal plates and water permeable sheets is subjected once or several times to high pressures in the order of several thousand p.s.i. This precompression step decreases the water content of the asbestos cement and greatly increases the density of the material and the mechanical strength thereof. After the compression process is completed the asbestos cement sheets are separated from the solid metal plates and the metal screens between which they had been sandwiched heretofore, and allowed to harden completely. The hardened sheets may be cut to strips 'with a carbide-tipped saw or an abrasive wheel and then machined to form tubular structures adapted to be used as casings of electric fuses.

The finished precompressed asbestos cement has a specific gravity of about lbs/cu. ft. and a moisture content of 5 to 13 percent of dry weight. Its water absorption exceeds that of many materials which have been used heretofore as materials for casings of electric fuses, but is not objectionable for most fuse casing applications. If desired the outer surface of fuse casings of precompressed liber-reinforced cement may be coated to close the pores thereof. The kind of coating material depends upon the maximum temperature to which the casing may be subjected when carrying the minimum fusing current for a suficiently long period of time to cause blowing of the fuse. Any alkaline-resistant type paint may be used as coating material. Sodium silicate, called water glass, may be used to close the pores of precompressed or densied ber reinforced cement, or heat resistant silicone paints may be used for that purpose.

If desired, certain inorganic additives, particularly siliceous materials, may be included in the mixture of asbestos and cement used to make the slurry from which precompressed or densified asbestos cement is made.

It will be understood from the foregoing that fuses according to this invention are only of interest in instances where their dimensions must be considerably less than those specified in the Underwriters Fuse Standard or, in other words, where miniaturized power fuses are needed. The choice of the casing material depends always upon the fuse structure it is intended to house. Only if the operating temperature of the fusible element is relatively high, and it is desired to arrange the surface of the casing unconventionally close tothe hot fusible element does the problem arise of substituting for any of another casing material.

It will also be apparent from the foregoing that while precompressed or densied inorganic fiber reinforced Portland cement is suiiiciently pressure-surge and heatshock resistant to be used for casings for miniaturized power fuses, the success of this particular application of precompressed or densified inorganic ber or asbestos cement is conditioned on minimizing the arc energy generated in the fuse. To this end the point of minimal cross-section of the fusible element must be so small as to result in a current-limiting ratio of less than 15.

While, in accordance with the patent statutes, we have disclosed the specific details of several embodiments of the invention, it is to be understood that these details are merely illustrative and that many variations thereof may be made without departing from the spirit and Iscope of the invention. It is our desire, therefore, that the language of the accompanying claims be interpreted as broadly as possible and that it be limited only as required by the prior state of the art.

We claim as our invention:

1. An electric high-interrupting-capacity cartridge fuse having a predetermined minimum fusing current and comprising a pair of terminal elements, a ribbon fuse link of -a high current-limiting action metal conductively interconnecting said pair of terminal elements, said ribbonfuse link having a minimum cross-sectional area ysufficiently small to result in a current-limiting ratio of less than 15, a body of quartz sand surrounding said fuse link, and a tubular casing supporting said terminal elements on the ends thereof and housing said fuse link and said body of quartz sand, said casing consisting of precompressed inorganic ber reinforced Portland cement and the lateral wall of said casing being arranged so close t said fuse link that the center of said lateral wall exceeds 120 deg. C. when said fuse link is carrying said minimum fusing current for the period of time required to cause fusion of said fuse link.

2. An electric high-interrupting-capacity cartridge fuse comprising a pair of terminal elements, a fuse link of silver conductively interconnecting said pair of terminal elements and defining a plurality of serially related points of reduced cross-section, said fuse'link being adapted to References Cited by the Examiner UNITED STATES PATENTS 2,270,723 1/ 1942 Boehne 200-144 2,801,672 8/1957 Baldwin et al. 200-144 2,834,852 5/1958 Swain et al 200--117 3,009,041 11/1961 Zlupko 200--144 3,029,328 4/1962 Kozacka 200-120 3,189,712 6/1965 Kozacka 200-120 OTHER REFERENCES Johns-Manville (I), Transite, publication No. IN 184A, .lune 1959, pp. 2-4.

Johns-Manville (Il), New Asbestos Ebony and Ohmstone, publication No. IN-229A, December 1959, pp. 2 and 3.

BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

1. AN ELECTRIC HIGH-INTERRUPTING-CAPACITY CARTRIDGE FUSE HAVING A PREDETERMINED MINIMUM FUSING CURRENT AND COMPRISING A PAIR OF TERMINAL ELEMENTS, A RIBBON FUSE LINK OF A HIGH CURRENT-LIMITING ACTION METAL CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS, SAID RIBBONFUSE LINK HAVING A MINIMUM CROSS-SECTIONAL AREA SUFFICIENTLY SMALL TO RESULT IN A CURRENT-LIMITING RATIO OF LESS THAN 15, A BODY OF QUARTZ SAND SURROUNDING SAID FUSE LINK, AND A TUBULAR CASING SUPPORTING SAID TERMINAL ELEMENTS ON THE ENDS THEREOF AND HOUSING SAID FUSE LINK AND SAID BODY OF QUARTZ SAND, SAID CASING CONSISTING OF PRECOMPRESSED INORGANIC FIBER REINFORCED PORTLAND CEMENT AND THE LATERAL WALL OF SAID CASING BEING ARRANGED SO CLOSE TO SAID FUSE LINK THAT THE CENTER OF SAID LATERAL WALLS EXCEEDS 120 DEG. C. WHEN SAID FUSE LINK IS CARRYING 